The advantages of providing pacing therapies to both the right and left heart chambers are well established. For example, in four chamber pacing systems, four pacing leads, typically bipolar leads, are positioned for both pacing and sensing in the respective heart chambers. To provide left side stimulation and sensing, leads are transvenously implanted in the coronary sinus region, for example, in a vein such as the great cardiac vein or the left posterior ventricular (LPV) vein proximate the left ventricle of the heart, or any other vein deemed suitable. Such placement avoids the risks associated with implanting a lead directly within the left ventricle which can increase the potential for the formation of blood clots which may become dislodged and then carried to the brain where even a small embolism could cause a stroke. As used herein, the phrase “coronary sinus region” refers to the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other vein accessible by way of the coronary sinus. The coronary sinus drains into the right atrium through the coronary sinus ostium or os.
The tip electrode of a lead implanted in the coronary sinus region can pace and sense left side ventricular activity. When such a lead includes a second electrode proximal of the tip electrode and residing in the coronary sinus above the left ventricle closely adjacent to the left atrium of the heart, pacing and sensing of left atrial activity is made possible. Moreover, the lead may include one or more electrodes for the delivery of electrical shocks for terminating tachycardia and/or fibrillation. Such cardioverting/defibrillating electrodes may be used by themselves or may be combined with pacing and/or sensing electrodes.
The implantation of a lead in the coronary sinus region is often difficult because of the extreme curvatures in the coronary vessels, their narrowness, anomalies in the vascular anatomy because of disease, and the number of veins which may communicate with the desired lead placement path. To facilitate placement of a lead in the coronary sinus region, an introducer sheath and dilator, often in combination with a stylet and/or guide wire, are conventionally used.
FIG. 1 is a side view of an introducer sheath 10. The introducer sheath has a main body 12 having a distal, tapering end 14 and a proximal end carrying a handle 16. The introducer sheath has a central passage 18 extending the entire length of the sheath. The passage 18 allows various objects, such as a dilator or a guide wire, to be inserted through the sheath. The introducer sheath has a length ranging, for example, from about 35 cm to about 70 cm, so that the distal end 14 of the introducer reaches the coronary sinus os. The introducer may further be configured to curve past the coronary sinus os either by constructing the distal end of the introducer to be flexible enough for a pre-curved guide wire to guide the introducer into the coronary sinus os or by pre-shaping the distal end of the introducer.
The tapered distal end 14 is substantially conical in shape converging to a circular edge 20 at the distal extremity. The introducer sheath may be made of any biostable, biocompatible material such as polyethylene and may comprise a peel-away structure of the kind well known in the art.
FIG. 2 is a side view of the introducer sheath 10 showing a dilator 30 fully inserted therein. The dilator 30 is typically a tubular member comprising a tapered distal end 32 and a knob 34 attached to the proximal end. Typically, the dilator will be somewhat stiffer than the introducer sheath. The dilator may have a length in the range of about 40–90 cm. The tapered distal end 32 of the dilator terminates at a point 36. The knob 34 at the proximal end of the dilator allows a clinician to manipulate the dilator during its insertion into or removal from the introducer sheath.
When inserted into the introducer sheath 10, the dilator 30 provides increased stiffness to the introducer sheath during introduction or insertion into tissue. The tip 36 of the tapered end of the dilator allows for easy entry through the skin into the subclavian or cephalic vein. By advancing the knob 34 relative to the introducer handle 16 and into engagement therewith, the dilator may be fully inserted into the introducer sheath. When fully inserted inside the introducer sheath, a length of about 1–10 cm of the dilator remains extended through and out of the distal end of the introducer sheath.
A typical, presently used method of implanting a lead using the introducer sheath and dilator is as follows:
Following standard Seldinger technique, an 18-gauge needle (not shown) is inserted into a vein (subclavian or cephalic). A J-tipped guide wire (not shown) is placed into the needle and advanced into the vein through the needle. The needle is then removed over the guide wire. The dilator, while located within the introducer, is pushed over the J-tipped guide wire through the tissue and ultimately into the vein and finally into the superior vena cava (SVC). At this point, the dilator and the J-tipped guide wire are pulled out of the introducer. The open end of the introducer is then placed in the right atrium near the coronary sinus os and the sheath is maneuvered as necessary to pass the tapered end thereof through the os. Once the introducer sheath is in place, a guide wire is inserted into the introducer sheath and advanced until the guide wire is inside the coronary sinus. The guide wire may be further advanced and maneuvered within the coronary sinus until the guide wire reaches a target vessel such as the LPV vein. The lead is then slid over the guide wire and advanced inside the introducer sheath until the distal end portion of the lead is inside the coronary sinus. While keeping the guide wire in the desired location (e.g., the LPV vein), the lead is further advanced along the guide wire for placement of the distal end portion thereof in the LPV vein. After placement of the lead, the guide wire is withdrawn from the lead followed by the removal of the introducer sheath. Alternatively, the lead may be placed using a stylet without a guide wire.
The introducer sheath is directed through the coronary sinus os with the aid of a fluoroscope. Nevertheless, the location of the coronary sinus os is difficult to ascertain and steering the introducer through the coronary sinus os is a difficult maneuver much dependent upon operator skill.
Obturators inserted within an introducer sheath have been used to facilitate passage of the introducer through the coronary sinus os. However, in current systems designed to deliver leads into the coronary sinus the distal portion of the obturator is pre-curved and accordingly its shape is thereby fixed. In many instances this does not provide adequate control to cannulate the coronary sinus. Further, because a substantial clearance typically exists between the outer surface of the distal portion of the obturator and the central passage of the introducer sheath, the edge of the tapered distal end of the sheath (such as the edge 20 in FIG. 1) may be disposed to engage cardiac tissue as the distal end of the introducer sheath is maneuvered into the coronary sinus.